1. Field of the Invention
The present invention relates to a process for manufacturing a semiconductor device. More particularly, the invention relates to a process for manufacturing a semiconductor device having a buried electrically conductive layer made of copper which has high resistance to electromigration and formed by a metal organic chemical vapor deposition method.
2. Description of Related Arts
In recent years, copper has been attracting attention as a material for wiring layers in semiconductor devices because of its low resistance and high electromigration resistance. As shown in FIG. 3, a wiring layer is formed by forming a trench 5 for wiring (or a contact hole) on an insulating film 2 which has been formed on a semiconductor substrate 1, depositing a thin barrier film 3 to prevent diffusion of copper into the insulating film 2 and the semiconductor substrate 1, and deposing a copper film on the barrier film 3 by a metal organic chemical vapor deposition (MOCVD) method.
Examples of organometallic compounds used in the MOCVD method for forming such copper films are hexafluoro-acetylacetonate copper (I) trimethylsilane and hexafluoro-acetylacetonate copper (I) triethoxyvinylsilane, which are liquid at room temperature and vaporize at 60.degree. C. to 90.degree. C.
The organometallic compound in a liquid state is fed in a vaporization chamber at a controlled flow rate, for example, of 0.1 to 2.0 ml/sec, by means of a liquid pump. The vaporization chamber is maintained at a constant temperature, for example, of 60.degree. C. to 90.degree. C., and the organometallic compound is vaporized in the chamber.
The vaporized organometallic compound is sent to a reaction chamber with a carrier gas, for example, of He or H.sub.2 gas at a flow rate of 50 cc/min to 200 cc/min. In the reaction chamber, a semiconductor substrate heated to 180.degree. C. to 250.degree. C. by being placed on a susceptor heated to a fixed temperature by electrical resistance is exposed to the vaporized organometallic compound.
The organometallic compound sent into the reaction chamber is decomposed by thermal energy from the semiconductor substrate. As a result, a copper film of copper atoms deposits on the substrate, while other products than copper atoms which are generated by decomposition are discharged.
The deposition rate of the copper film at this time depends greatly on the temperature of the semiconductor substrate. Within the above-mentioned temperature range, the higher the temperature is, the higher the deposition rate is. However, the higher the temperature is, the more difficult it is to cover a step defined by the trench for wiring. A reference, Thin Solid Films 262(1995), pp. 12-19, describes that the deposition of a copper film at low temperatures results in better step coverage.
However, a problem lies in that, if the temperature of the substrate is set low during the deposition of copper films with a view to improving step coverage, throughput of thin copper film formation does not increase.
On the other hand, if the temperature of the substrate is raised for the purpose of improving the throughput, the step coverage becomes worse. As a result, voids may possibly be formed in the trench for wiring or the contact hole. This gives rise to problems such as increased resistance and adverse effect on reliability of devices. In other words, both the deposition rate of the copper film and the step coverage depend greatly on temperature, and they are in a trade-off relation. Therefore, it has been impossible to realize a good deposition rate and good step coverage at the same time.
Also conventionally, since the semiconductor substrate is heated by being put on the susceptor heated by electrical resistance, it has been difficult to finely adjust the temperature of the substrate and it has taken much time to control the temperature of the substrate.
Furthermore, the copper film may possibly be deposited on other places than the substrate, e.g., on the susceptor.